The present invention relates to a new and improved method for the continuous casting of steel strands, especially slabs, wherein the liquid core or pool is stirred by the action of electromagnetic forces at the region of the continuous casting mold.
During the continuous casting of steel, especially larger strand sectional shapes or formats, there arise during the course of the casting time fluctuations or changes in the withdrawal speed of the cast strand, which are predicated upon different causes. Thus, it is known in conventional practice to slow down the withdrawal speed of the strand or even to reduce such to null i.e. to stop the strand at the end of the casting or pouring operation. Thereafter the slag is removed from the molten level or meniscus of the strand end still located in the continuous casting mold and water is sprayed so that the steel at the region of the bath level or meniscus is solidified. Below this solidified bath level there is, however, formed a hollow space due to the shrinkage effects of the strand. Due to the formation of bridges or the like, particularly between the wide sides of the mold when casting slab formats or sectional shapes, there is hindered any further flow of the molten metal, so that there can be formed hollow spaces which extend through a length of over one meter. Since this length must be cut off before further processing of the strand there arise appreciable losses in the ouput or yield of the continuously cast strands.
Further fluctuations in the strand withdrawal speed arise, for instance, when performing sequential pours or casting operations, during which the teeming or pouring operation is slowed down when there is accomplished a change in the ladle or tundish. Moreover, changes in the withdrawal speed of the continuously cast strand arise when altering the slab width by adjusting the narrow sides of the mold during the continuous casting operation or when there occurs metal break-out, or when there arise other disturbances in the casting operation during which the strand must be stopped.
It is generally known in the continuous casting art to stir the liquid core or pool within the solidified strand shell or skin at the region of the continuous casting mold by stirrers arranged at the height of the continuous casting mold or therebelow. These stirrers stir the liquid pool of the cast strand by the application of electromagnetic forces. During the casting events discussed above, where as previously explained there arises a slowing down of the withdrawal speed or a stopping of the continuously cast strand, the residence time of the cast strand within the effective region of the electromagnetic stirrer is greater than during the time of the fully effective static casting operation.
Now if the strand, upon the occurrence of such operating events, is stirred in the same manner as during normal operations then there arises a local "over-stirring" of the continuously cast strand, i.e. it is stirred for much too great amount of time at the same section of the cast strand. In this case when etching a cross-section of the strand or taking of a sulphur print of the strand cross-section, there has been noticed a pronounced transition between the outer zone which has already solidified at the time of stirring and the inner situated globulitic zone. This pronounced transition is optically discernible at the rolled product, and therefore undesired and impairs the quality of the casting. Moreover, also at the end of the casting operation difficulties can therefore arise because the intentional solidification of the metal located at the region of the meniscus or bath level is disturbed, because the thin formed strand shell at the end of the strand tends to melt by virtue of the flow of hot steel which has been moved to the region of the strand shell by virtue of the pronounced stirring or agitation of the molten metal. This increases the danger of a metal break-out during the subsequent withdrawal of the cast strand.